One-Pot Four-Segment Ligation Using Seleno- and Thioesters: Synthesis of Superoxide Dismutase

Mechanochemical synthesis of organoselenium compounds

We disclose herein a strategy for the rapid synthesis of versatile organoselenium compounds under mild conditions. In this work, magnesium-based selenium nucleophiles are formed in situ from easily available organic halides, magnesium metal, and elemental selenium via mechanical stimulation. This process occurs under liquid-assisted grinding (LAG) conditions, requires no complicated pre-activation procedures, and operates broadly across a diverse range of aryl, heteroaryl, and alkyl substrates. In this work, symmetrical diselenides are efficiently obtained after work-up in the air, while one-pot nucleophilic addition reactions with various electrophiles allow the comprehensive synthesis of unsymmetrical monoselenides with high functional group tolerance. Notably, the method is applied to regioselective selenylation reactions of diiodoarenes and polyaromatic aryl halides that are difficult to operate via solution approaches. Besides selenium, elemental sulfur and tellurium are also competent in this process, which showcases the potential of the methodology for the facile synthesis of organochalcogen compounds.

Quinoline-Based Photolabile Protection Strategy Facilitates Efficient Protein Assembly

Native chemical ligation (NCL) provides a powerful solution to assemble proteins with precise chemical features, which enables a detailed investigation of the protein structure-function relationship. As an extension to NCL, the discovery of desulfurization and expressed protein ligation (EPL) techniques has greatly expanded the efficient access to large or challenging protein sequences via chemical ligations. Despite its superior reliability, the NCL-desulfurization protocol requires orthogonal protection strategies to allow selective desulfurization in the presence of native Cys, which is crucial to its synthetic application. In contrast to traditional thiol protecting groups, photolabile protecting groups (PPGs), which are removed upon irradiation, simplify protein assembly and therefore provide minimal perturbation to the peptide scaffold. However, current PPG strategies are mainly limited to nitro-benzyl derivatives, which are incompatible with NCL-desulfurization. Herein, we present for the first time that quinoline-based PPG for cysteine can facilitate various ligation strategies, including iterative NCL and EPL-desulfurization methods. 7-(Piperazin-1-yl)-2-(methyl)quinolinyl (PPZQ) caging of multiple cysteine residues within the protein sequence can be readily introduced via late-stage modification, while the traceless removal of PPZQ is highly efficient via photolysis in an aqueous buffer. In addition, the PPZQ group is compatible with radical desulfurization. The efficiency of this strategy has been highlighted by the synthesis of γ-synuclein and phosphorylated cystatin-S via one-pot iterative ligation and EPL-desulfurization methods. Besides, successful sextuple protection and deprotection of the expressed Interleukin-34 fragment demonstrate the great potential of this strategy in protein caging/uncaging investigations.

Chemical synthesis of ferredoxin with 4 selenocysteine residues using a segment condensation method

Ferredoxin (Fd) is an electron carrier protein containing a [2Fe-2S] cluster. In this paper, we synthesized Se-Fd, in which four Cys residues coordinated to the cluster are substituted to selenocysteine. After the one-pot segment coupling by the thioester method, followed by deprotection and cluster loading, the desired Se-Fd was successfully obtained.

Peptidyl ω-Asp Selenoesters Enable Efficient Synthesis of N-Linked Glycopeptides

Chemical synthesis is an attractive approach allows for the assembly of homogeneous complex N-linked glycopeptides and glycoproteins, but the limited coupling efficiency between glycans and peptides hampered the synthesis and research in the related field. Herein we developed an alternative glycosylation to construct N-linked glycopeptide via efficient selenoester-assisted aminolysis, which employs the peptidyl ω-asparagine selenoester and unprotected glycosylamine to perform rapid amide-bond ligation. This glycosylation strategy is highly compatible with the free carboxylic acids and hydroxyl groups of peptides and carbohydrates, and readily available for the assembly of structure-defined homogeneous N-linked glycopeptides, such as segments derived from glycoprotein EPO and IL-5.

Untapped Opportunities of Resin-to-Resin Transfer Reactions (RRTR) for the Convergent Assembly of Multivalent Peptide Conjugates

Handling of the individual fragments remains a bottleneck in the convergent assembly of peptides. Overlooked since the emergence of ligation chemistries during the past two decades, so-called resin-to-resin transfer reactions (RRTR) are here described as a strategic shortcut in this context. Condensation of the involved moieties at an acceptor resin is facilitated by shuttling peptide segments directly from a donor resin in a one-pot fashion. The straightforward synthesis of a sterically constrained 13-mer peptidosteroid model illustrates the utility of this approach, presenting the first successful application of the RRTR methodology in the field of multivalent design and bioconjugation. Relying on established procedures to generate, monitor and isolate intermediates and products, the solid-phase nature of the entire strategy allows for the fast construction of polypeptide adducts and libraries thereof. As such, a rejuvenated use and new opportunities for RRTR are reported.

Mechanism and chemoselectivity origins of bioconjugation of cysteine with Au(iii)-aryl reagents

A detailed computational study is presented on the reaction mechanism of selective cysteine S-arylation by cationic Au(iii)-aryl reagents. The chemoselectivity origins have been elucidated through comparison with potential N- and O-arylation, showing that the acidity and nucleophilicity of the residue are two inherent controlling factors.

One-pot multi-segment condensation strategies for chemical protein synthesis

This paper describes recent advances of one-pot multi-segment condensation strategies based on kinetically controlled strategies and/or protecting group-removal strategies in chemical protein synthesis.

Triple Function of 4-Mercaptophenylacetic Acid Promotes One-Pot Multiple Peptide Ligation

One-pot multiple peptide ligation is a key technology to improve the efficiency of chemical protein synthesis. One-pot repetitive peptide ligation requires a cycle of three steps: peptide ligation, removal of a protecting group, and inactivation of the deprotection reagent. However, previous strategies are not sufficient because of harsh deprotection conditions, slow deprotection rates, and difficulty in quenching the deprotection reagent. To address these issues, we developed a rapid, efficient deprotection and subsequent quenching strategy using an allyloxycarbonyl group to protect the N-terminal cysteine residue. 4-Mercaptophenylacetic acid (MPAA), a thiol additive for native chemical ligation, functioned not only as a scavenger for π-allyl palladium complexes, but also as a quencher of palladium(0) complexes. By utilizing the multifunctionality of MPAA, we carried out a one-pot five-segment ligation to afford histone H2AX (142 amino acids), which was isolated in 59 % yield.

Redox-Neutral Synthesis of Selenoesters by Oxyarylation of Selenoalkynes under Mild Conditions

An approach for the mild synthesis of selenoesters starting from selenoalkynes through an acid-catalyzed, redox-neutral oxyarylation reaction is reported. Brønsted acid activation of a selenoalkyne leads to a selenium-stabilized vinyl cation, which is captured by an aryl sulfoxide and undergoes sigmatropic rearrangement to deliver the final α-arylated selenoester product. Computational studies have been carried out to elucidate the nature of the Se-stabilized carbocation.

Synthesis and semisynthesis of selenopeptides and selenoproteins

The versatile chemistry of the genetically encoded amino acid selenocysteine (Sec) is employed in Nature to expand the reactivity of enzymes. In addition to, its role in biology, Sec is used in protein engineering to modify folding, stability, and reactivity of proteins, to introduce conjugations and to facilitate reactions. However, due to limitations related to Sec's insertion mechanism in Nature, much of the production of Sec containing peptides and proteins relies on synthesis and semisynthesis. Here, we review recent advances that have enabled the assembly of complicated selenoproteins, including novel uses of protecting groups for solid phase peptide synthesis, rapid selenoester driven chemical ligations and versatile expressed protein ligations.